In optical disc technologies, data can be read out from a rotating optical disc by irradiating the disc with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disc.
On a read-only optical disc, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disc. On the other hand, on a rewritable or write once optical disc, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a substrate on which tracks with spiral lands or grooves are arranged. A phase change type recording film is used for a rewritable optical disc but an organic dye material film or any other suitable material may be used for a write once optical disc.
In writing data on a rewritable or write-once optical disc, data is written there by irradiating the optical disc with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the tracks and the thickness of the recording material film are both smaller than the thickness of the optical disc base material. For that reason, those portions of the optical disc, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” actually has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information layer”. Every optical disc has at least one such information layer. Optionally, a single information layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on a rewritable or write-once optical disc or to write data on such an optical disc, the light beam always needs to maintain a predetermined converging state on a target track on an information layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information layer such that the focus position of the light beam is always located on the information layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disc (which direction will be referred to herein as a “disc radial direction”) such that the light beam spot is always located right on a target track.
Various types of optical discs such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Meanwhile, CDs (compact discs) are still popular now. Recently, there is a growing demand for optical discs with storage capacities that are big enough to store high-definition data there. To meet such a demand, a Blu-ray Disc (which will be referred to herein as a “BD”) was developed. A BD-RE has already been available for an affordable price as a rewritable disc, but a BD-R disc, which is a write-once disc that can be produced at a lower cost than a BD-RE, is also under development. In the meantime, an HD-DVD (high definition DVD) that complies with a different set of standards is still being developed.
Each of these optical discs such as CDs, DVDs and BDs has a principal surface (i.e., light-incoming side) and a back surface (i.e., a label side) and includes at least one information layer between them. Every optical disc has an overall thickness of approximately 1.2 mm and a diameter of either 12 cm or 8 cm.
A CD's information layer is located at a depth of about 1.1 mm as measured from the principal surface. To read data from the CD's information layer, a near-infrared laser beam (with a wavelength of 785 nm) needs to be converged such that its focal point is located right on the information layer by focus control. An objective lens for use to converge the light beam needs to have a numerical aperture (NA) of approximately 0.45. A DVD's information layer is located at a depth of approximately 0.6 mm as measured from the principal surface. In an actual DVD, two substrates, each having a thickness of approximately 0.6 mm, are boned together with an adhesive layer. In an optical disc with two information layers, the respective distances from the principal surface 2 to the information layers are approximately 0.57 mm and approximately 0.63 mm, respectively. That is to say, those two information layers are located very close to each other. To read and write data from/on the DVD's information layer, a red laser beam (with a wavelength of 660 nm) needs to be converged such that its focal point is located right on the information layer by focus control. An objective lens for use to converge the light beam needs to have a numerical aperture (NA) of approximately 0.6.
On the other hand, a BD includes a thin coating layer (light transmitting layer) with a thickness of 100 μm on the principal surface and its information layer is located at a depth of about 0.1 mm as measured from the principal surface. To read data from the BD's information layer, a blue violet laser beam (with a wavelength of 405 nm) needs to be converged such that its focal point is located right on the information layer by focus control. An objective lens for use to converge the light beam needs to have a numerical aperture (NA) of approximately 0.85. Meanwhile, an HD-DVD has a cross-sectional structure similar to that of a DVD and its information storage layer is located at a depth of about 0.6 mm as measured from the principal surface. To read data from the HD-DVD's information layer, a blue violet laser beam (with a wavelength of 405 nm) needs to be used as in BDs and an objective lens for use to converge the light beam should have a numerical aperture (NA) of 0.65 according to recently proposed specifications.
Currently, these various types of optical discs are on the market and used extensively. Under the circumstances like these, a single optical disc drive should read from, and write to, as many types of optical discs as possible. For that purpose, the optical disc drive should include a light source and an optical system, both of which can deal with multiple types of optical discs, and should appropriately recognize the type of the optical disc that has been loaded into the optical disc drive.
Meanwhile, there are optical discs with multiple information layers. FIG. 11 is a perspective view schematically illustrating the configuration of a dual-layer optical disc. Specifically, the optical disc 25 shown in FIG. 11 is a dual-layer optical disc including a first information layer 21 and a second information layer 22. More specifically, the optical disc 25 includes the first and second information layers 21, 22, a base member 24 to support these information layers 21, 22 and a protective coating 23 that covers the first information layer 21.
To read data from the first information layer 21 of this optical disc 25, a focus control needs to be carried out such that the focal point of the light beam is located on the first information layer 21. However, to get ready to read data from the second information layer 22 next while reading data from the first information layer 21, the focus position of the light beam needs to be shifted from the first information layer 21 to the second information layer 22. Such shift of the focus position will be referred to herein as a “focus jump”. And to carry out the focus jump, the objective lens that converges the light beam needs to be moved perpendicularly to the information layers of the optical disc.
When the focus position of a light beam moves between information layers to carry out the focus jump, the absolute value of a focus error signal increases. That is why the focus jump should be started after the focus control has once been put on hold, or suspended. And the focus control needs to be resumed when the focal point of the light beam comes sufficiently close to the target information layer by moving the objective lens.
FIG. 12 is a graph showing a focus error signal generated from a dual-layer optical disc. In FIG. 12, the abscissa represents the position of an objective lens with respect to that optical disc and the ordinate represents the value of a focus error signal. Portions (a) through (c) of FIG. 13 are timing charts showing the timing to make a focus jump for a conventional optical disc drive. Specifically, portion (a) of FIG. 13 shows the waveform of a focus error signal during a focus jump operation. Portion (b) of FIG. 13 shows the waveform of a focus drive signal during the focus jump operation. And portion (a) of FIG. 13 is a timing chart showing the timings to turn ON and OFF the focus control during the focus jump operation.
While the objective lens is gradually moved toward the optical disc 25 shown in FIG. 11, two S-curves S1 and S2 are soon generated on the focus error signal as shown in FIG. 12. The points A and B shown in FIG. 12 represent the in-focus positions of the respective information layers.
Specifically, the S-curve S1 is generated from the first information layer 21 of the optical disc 25 shown in FIG. 11, while the S-curve S2 is generated from the second information layer 22 thereof. While a read operation is being performed on the first information layer 21, the objective lens is located at a position corresponding to the point A on the S-curve S1. If a focus jump is made in such a state, the focus error signal passes through the S-curve S1 of the first information layer 21 and an intermediate layer range 26, and then reaches a point C of the S-curve S2 generated from the second information layer 22. After that, the S-curve S2 is observed in the direction leading from the point C toward the point B.
Ideally, when the objective lens reaches a position corresponding to the point C, the objective lens is preferably braked by a focus actuator and controlled so as to stop at a position corresponding to the in-focus position B of the S-curve S2 generated from the second information layer 22. In the intermediate layer range 26, however, there could be some noise, of which the amplitude is approximately 10% of the peak value of the S-curves.
That is why even when a focus jump is made from the first information layer 21 toward the second information layer 22, it is not clear exactly when to brake the objective lens and it is difficult to stop the objective lens at the position corresponding to the in-focus position B.
For that reason, the focus control is sometimes turned ON by braking the objective lens that is located at a position corresponding to an intermediate point between the points C and B of the S-curve S2. That is to say, the objective lens may sometimes be braked after the point D has been passed and before the peak value of the S-curve S2 is reached. Or the objective lens may start being braked at the point D and finish being braked at a predetermined position between the peak value of the S-curve S2 and the point B.
First, the focus control is turned OFF at a time ta as shown in portion (c) of FIG. 13 and then an accelerating pulse is output to a focus actuator driver (not shown) from the time ta through a time tb as shown in portion (b) of FIG. 13. In the meantime, the objective lens is moved toward the optical disc. And when the focus error signal reaches a level fd at a point D (and at a time td) after having passed the point C as shown in portion (a) of FIG. 13, a decelerating pulse (i.e., a braking pulse) is output to a focus actuator driver. After that, the objective lens gradually slows down and then stops in response to a decelerating pulse at a time te, when the focus control is turned ON again and the focus jump is completed.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2000-200431 (Paragraphs #22 and #23 and FIG. 5)        